This application relates to color cathode ray tubes, and more particularly to an improvement which makes possible a reduction in the cost of certain components of a color CRT (cathode ray tube) of the shadow mask type. Color cathode ray tubes today include a number of auxiliary components which are deemed to be necessary to provide, with a high degree of personal safety and under a wide variety of operating conditions, the high quality pictures which the consuming public demands.
It is common practice to provide on a color CRT a magnetic shield for shielding the interior of the bulb from the earth's magnetic field and other stray ambient fields. It is known to locate the magnetic shield in the bulb's interior--see U.S. Pat. Nos. 3,549,932-Lindeman and 3,822,453-Shrader. In other prior art disclosures a magnetic shield is located on the outside of the bulb--see U.S. Pat. No. 3,422,220-Bathelt et al; U.S. Pat. No. 3,417,201-Joseph et al; U.S. Pat. No. 2,890,362-Francken, and U.S. Pat. No. 3,430,985-Ammerman et al. See also Color Television Picture Tubes by Morrell et al, Academic Press, 1974, pp. 112-114. The external shield is the more popular arrangement in commercial use.
Another common color CRT component is a "degaussing" coil provided to insure that the purity of the colors reproduced on the screen of the tube are not degraded by the effects of the earth's magnetic field or other stray ambient magnetic fields. Typically a degaussing coil is mounted so as to surround the color CRT bulb immediately rearwardly of the screen area. A decaying sinusoidal electrical current is passed through the coil, usually as an automatic consequence of the initiation of tube operation or some other predictable event.
The effect of energizing the degaussing coil is to demagnetize the magnetic shield (if any) and the shadow mask assembly, as well as any other magnetizable structures in the tube which have picked up a spurious magnetism capable of diverting the electron beams from their intended flight path and thus cause color impurity in the reproduced pictures. The degaussing coil is connected at each end to a degaussing circuit in the chassis of the television receiver, from which circuit it receives an appropriate degaussing current. Typical U.S. patents depicting prior art degaussing structures are: U.S. Pat. No. 3,322,998-Norley; 3,324,343-Norley; 3,340,433-Reith et al; 3,344,307-Van Anrooy et al; 3,404,307-Hayden; 3,433,993-Norley; 3,657,729-Hansen et al; 3,699,400-Marsh, Jr. See also the Morrell et al text at pp. 112-114.
The prior art also discloses aggregating into a single assembly degaussing coils and an external, or internal-external magnetic shield; see U.S. Pat. No. 3,340,417-Panis et al and U.S. Pat. No. 3,867,668-Shrader.
A patent to Brownell U.S. Pat. No. 2,907,914 discloses for a round-type color CRT a coiled strip magnetic shield designed to permit passage of a D.C. current for the purpose of causing the strip "to operate at a higher point on its magnetization curve and thus effectively increase the permeability of the shielding material." The strip also serves, when energized with a D.C. current, as a "field neutralizing coil." In this connection see also British Pat. No. 1,129,167 and U.S. Pat. No. 2,860,328-Langworthy.
Further, it is necessary to prevent the bulb from imploding should the bulb be fractured by a missile or otherwise be suddenly devacuated. Conventionally, color television picture tubes have an evacuated glass bulb which includes a faceplate with a rearward flange and a funnel sealed to the faceplate flange along a planar sealing interface. The faceplate has a concave inner surface upon which is deposited a cathodoluminescent phosphor screen. Due to the high vacuum in the bulb, several tons of atmospheric pressure are exerted on the faceplate causing the bulb to be susceptible to implosion (The term "implosion" is defined by Underwriter's Laboratory Incorporated as a "rapid and sudden inward bursting of a high-vacuum glass envelope.") It is of the utmost importance in the interest of safety to prevent the bulb from violently imploding should, for example, it be struck by a heavy missile.
There have evolved a number of approaches to implosion protecting color CRT's (cathode ray tubes) of the described type having a conventional bulb (with a flanged faceplate). One approach is implemented in systems referred to as "rimbond" systems. The rimbond system has a metal frame which surrounds the flange found on every conventional faceplate. A small gap between the frame and the faceplate flange is filled with a cement--typically an epoxy resin. In a rimbond system, the frame is not under tension. The cement contains the glass shards of a shattered faceplate long enough to permit gradual (and therefore nonviolent) devacuation of the bulb. Exemplary U.S. patents illustrating rimbond systems are U.S. Pat. Nos. 3,485,407; 3,558,818; 3,412,203 and 3,835,250. A major drawback to rimbond systems has been the large amount of (very costly) epoxy cement required.
A second basic implosion protection approach is termed the "tension band" approach. Systems implementing this approach comprise a band which is placed around the faceplate flange and put under very high tension. The tension band around the faceplate flange constricts the flange and, like the rimbond systems, holds the shards of a fractured faceplate in position until the bulb has gradually-devacuated. Numerous patents have been issued on various aspects of tension band systems. See U.S. Pat. Nos. 3,818,557; 3,456,076; 3,556,306; 3,597,537; 3,777,057; 3,845,530; 3,890,464; 3,332,564; 3,220,593; and 3,332,570.
Another approach is to bond a transparent protective shield over the front surface of the faceplate. Systems following this approach are commonly termed "bonded panel" systems. See the Morrell et al text. pp. 111-112.
Yet another approach to implosion-protecting color CRT's with conventional faceplates involves using an expansible frame and a tension band to constrain the expansibility of the frame. The frame comprises an overlapped air of "C"-shaped half-frames. A viscous epoxy cement is typically applied to each of the half-frames; the half-frames are then placed around the faceplate flange with their ends overlapping. Before the cement has set, a tension band is drawn up tightly around the frame and faceplate flange. It is common to use a lighter weight tension band than is used in a pure tension band system. See U.S. Pat. No. 3,845,530-Platt.
A U.S. Pat. No. 3,647,960-Takemoto et al discloses an implosion protection system for a color CRT which comprises a series of closely spaced, mutually insulated turns of wire wrapped around the flange of the faceplate. An electrical current is passed through the wire to polymerize a polymerizable coating on the wire.
A patent to Powell et al-U.S. Pat. No. 3,519,161 discloses the use of one or a limited number of closed loops of reinforcing wire around the flange of a faceplate of a cathode ray tube of the conventional type to enhance the resistance of the tube to implosion. The wire loops may be tensed or relaxed. The loops may be covered by a layer of glass or epoxy cement.
U.S. Pat. No. 3,166,211-Stel et al discloses an implosion protection system for a CRT comprising a fiber-impregnated sheath on the exterior of the CRT bulb. In one embodiment, glass fibers are embedded in a cord which is wrapped around and bound to the exterior of a tube to provide implosion protection. The following patents disclose implosion systems for flangeless faceplate tubes: U.S. Pat. Nos. 4,004,092-Rogers; 4,016,364-Rogers; 4,021,850-Rogers; 4,084,193-Palac; 4,037,255-Hill; and 4,054,913-Lerner. U.S. Pat. No. 4,084,193 teaches the use of a tension band in the form of a tightly wound coil of wire.
A U.S. Pat. No. 2,222,197 to Engels discloses a CRT in which the bulb comprises a flangeless faceplate inset in an expanded open end of a cooperating funnel. A band allegedly providing implosion protection surrounds the outside of the funnel near the open end thereof.
A fourth exterior component of a color CRT is the "outer conductor." To provide a large capacitor for filtering the high voltage applied to the CRT screen, electrically conductive coatings are applied to the inner and outer surfaces of the CRT funnel. It is taught in U.S. Pat. No. 3,614,519-Figlewicz et al that a high permeability foil jacket form-fitted to the outer surface of the funnel may serve as a combined magnetic shield and outer conductor.
A fifth necessary exterior CRT component is a system providing for mounting the tube in a television chassis. The above-identified patents disclose numerous types of mounting brackets and other structures for mounting a tube in a TV chassis.